DISCLAIMER: This is a working document provided by the Centers for Disease Control and Prevention (CDC) 

technical support contractor, SC&A, for use in discussions with the National Institute for Occupational Safety and 

Health (NIOSH) and the Advisory Board on Radiation and Worker Health (ABRWH), including its Working Groups 

or Subcommittees. Documents produced by SC&A, such as memorandum, white paper, draft or working documents 

are not final NIOSH or ABRWH products or positions, unless specifically marked as such. This document prepared 

by SC&A represents its preliminary evaluation on technical issues.

NOTICE: This document has been reviewed to identify and redact any information that is protected by the 

Privacy Act 5 USC §552a and has been cleared for distribution. 

MEMORANDUM

TO: Advisory Board on Radiation and Worker Health, Work Group on Metals and 

Controls Corp. 

FROM: Robert Anigstein, SC&A, Inc. 

DATE: July 26, 2019 

SUBJECT: Review of NIOSH’s “Metals and Controls Corp. Thorium and Welding Exposure 

Model” 

1 Background 

On April 8, 2019, the National Institute for Occupational Safety and Health (NIOSH) completed 

a white paper (McCloskey and Sharfi 2019) in response to two concerns raised by a petitioner for 

Special Exposure Cohort (SEC) Petition SEC-00236, which addressed the residual period at the 

Metals and Controls Corp. (M&C) in Attleboro, Massachusetts, from January 1, 1968, through 

March 21, 1997. These concerns included assessments of workers’ exposures during welding 

activities and exposures to residual thorium contamination. The white paper was distributed on 

May 10, 2019; SC&A was tasked with reviewing the white paper on May 14. 

2 Review of NIOSH White Paper

2.1 Internal Exposures to Thorium

McCloskey and Sharfi (2019) acknowledged that thorium operations occurred at M&C during 

the period that the facility was designated as an Atomic Weapons Employer (AWE). Their aim 

was to estimate internal doses to M&C workers from intakes of thorium during the residual 

period. 

2.1.1 Inventory of Uranium and Thorium

To evaluate worker exposures to thorium at M&C, McCloskey and Sharfi (2019) reviewed a 

1962 analysis of possible losses of nuclear materials and accompanying financial liabilities, 

performed on behalf of M&C in order to determine the need for insurance to cover such losses 

(ASTRA 1962). The report of this analysis included an inventory of uranium and thorium at 

M&C on January 1, 1962, reproduced in Table 1 of the present memo. As shown, the table 

includes separate listings for “Commission” and “License.” “Commission” presumably means 

the Atomic Energy Commission. The reason for the separate listings is not clear. According to 

McCloskey and Sharfi, the inventory comprised 244 kg of thorium and 7,097 kg of uranium. The 

cited thorium inventory is the total of the amounts listed under “Commission” and “License.” 

However, the amount of uranium cited is less than 7,854 kg, which is the total of the amounts in 
Memo – M&C Thorium and Welding Exposure Model 2 SC&A – July 26, 2019

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these two columns. Furthermore, according to the totals listed in Table 1, the mass of the total 

uranium inventory is 32 times that of thorium, not 29 times as cited by McCloskey and Sharfi. 

These discrepancies should be resolved. 

Table 1. Inventory of Special Nuclear Materials at M&C (January 1, 1962) (kg)

Material Commission License Commission + 

License

93% Enr. U 1,264 18 — 

20% Enr. U — 71 — 

3.2% Enr. U — 1,449 — 

2.2% Enr. U — 3,932 —

1.8% Enr. U — 363 — 

Normal U 61 192 —

Depleted U 158 346 — 

Total U 1,483 6,371 7,854

Thorium 198 46 244

Source: ASTRA (1992, Table 2)

Observation 1: The uranium inventory cited by NIOSH is inconsistent with that in the source 

document. 

We make this an observation rather than a finding, because the uranium inventory is not used in 

the proposed NIOSH dose methodology. 

2.1.2 Confirmation of Residual Thorium Contamination

McCloskey and Sharfi (2019) noted that Creative Pollution Solutions (CPS, n.d.) reported 

elevated levels of thorium during their 1992 survey of the burial area between Buildings 11 and 

12. SC&A observed that CPS reported a soil sample containing 110 pCi/g of 232Th and an equal 

concentration of 228Th (i.e., the two isotopes were in secular equilibrium). Since, according to 

Sowell (1985), the onsite burials took place in 1958–1961, the material was at least 31 years old 

at the time of the analysis. Even if the thorium had been freshly purified in 1961, 228Th would 

have been at ~96% of its secular equilibrium concentration at the time of the analysis, so these 

results are not surprising.

2.1.3 NIOSH Assessment of Uranium-to-Thorium Ratio

McCloskey and Sharfi (2019, p. 4) utilized the analyses of “samples taken of waste and materials 

removed from the former AWE areas and placed in the burial area” to determine an activity ratio 

of uranium to 232Th. The authors cited 754 samples that were analyzed for both uranium and 

232Th. “NIOSH determined a paired activity ratio of uranium to thorium-232 for each of these 

samples and calculated a geometric mean ratio of uranium to thorium-232 as 9.88:1.” As 

explained later in the white paper, NIOSH intended to apply this ratio to measured 

concentrations of uranium in pipe sediments to estimate the concurrent concentrations of 232Th. 
Memo – M&C Thorium and Welding Exposure Model 3 SC&A – July 26, 2019

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2.1.4 Reviewers’ Comments on NIOSH Assessment of Uranium-to-Thorium Ratio

We first note that McCloskey and Sharfi (2019) did not cite the source of the sampling data used 

to determine the activity ratio of uranium to 232Th. We presume that these data comprise 

radionuclide concentrations in soil samples listed by Sowell (1985, Tables 4, 5A, 6A, 11, 12A, 

and 13A), who reported 751 such samples, according to our count, vs. 754 samples cited by 

McCloskey and Sharfi. This minor discrepancy is most likely due to a clerical error—we believe 

that the authors utilized the Sowell data, which were cited in their previous report (McCloskey 

and Sharfi 2018).

A review of the Sowell (1985) data shows that the vast majority of the 232Th concentrations 

reflect typical levels of naturally occurring thorium in uncontaminated soils. To quantify this 

observation, we calculated the expected distribution of naturally occurring 232Th on the M&C 

site, based on concentrations in baseline soil samples collected off site in the Attleboro, 

Massachusetts, area listed by Sowell (1985, Table 1-B). The 232Th concentrations in the five 

samples have a geometric mean of 0.81 pCi/g and a geometric standard deviation of 1.49. 

Assuming an underlying lognormal distribution, the upper 95th percentile of this distribution is 

1.56 pCi/g, while the 99th percentile is 2.05 pCi/g. 

We note that Sowell (1985, Table 4) comprised 482 samples collected at the surface at grid-line 

intersections in the Building 12 burial area. If the 232Th concentrations were due to natural 

background levels, we would expect that 1% (4–5 samples) would have concentrations at or 

above the 99th percentile of the baseline samples. In fact, only one sample listed in Table 4, with 

a concentration of 2.12 pCi/g, was above this level. Similarly, Sowell (1985, Table 11) lists 139 

sampling results for soil samples collected at the surface at grid-line intersections in the outdoor 

area surrounding Building 10. With this number of samples, we would expect one to two 232Th

concentrations to be at or above the 99th percentile. In fact, the highest value is 7.18 pCi/g, with a 

second reading of 3.11 pCi/g. All other 232Th values are well below the 99th percentile. With the 

exception of these two readings, these values are consistent with natural background activity 

concentrations in soil. 

Reviewing the rest of Sowell’s (1985) data, we note that Table 12A, listing results for three 

samples collected from the top 10-cm soil layer at locations of elevated contact radiation levels

near Building 10, includes one reading of 1.70 pCi/g of 232Th, which is above the 95th percentile 

but below the 99th. However, a set of three samples cannot be used to establish an activity 

concentration ratio. Table 13A lists results for 24 borehole soil samples near Building 10. The 

highest 232Th concentration is 1.25 pCi/g, which is less than the 95th percentile, contrary to the 

expectation that 5% (one sample) would exceed the 95th percentile background. 

We therefore conclude that all the 232Th data in Sowell (1985, Tables 4, 11, 12A, and 13A), with 

the possible exception of two samples listed in Table 11, are consistent with naturally occurring 

232Th in soils in the Attleboro area and cannot be used to estimate intakes of 232Th originating 

from AWE activities. The only data that include a substantial number of samples with 232Th 

levels significantly elevated above background are those presented by Sowell (1985, Tables 5A 

and 6A). 
Memo – M&C Thorium and Welding Exposure Model 4 SC&A – July 26, 2019

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2.1.5 New NIOSH Bounding Method to Estimate Intakes of Thorium

McCloskey and Sharfi (2019), referring to their earlier report (McCloskey and Sharfi 2018), 

cited the 95th percentile uranium activity in the pipes under Building 10 as 6,888 pCi/g. They 

calculated the corresponding mass concentration of uranium, based on the specific activity of 

natural uranium. They assumed equal concentrations of natural uranium and 232Th by mass, 

based on the uranium-to232Th activity ratio of 9.88:1 cited in section 2.1.3 of the present memo, 

and on the relative specific activities of the two radionuclides. They concluded that uranium and 

232Th each comprised 1% of the pipe sediments and thus 1% of the airborne dust generated from 

these sediments. The authors calculated an effective dose commitment of 10.42 mrem/y via the 

inhalation pathway, based on one month of subsurface work during each year of the residual 

period. Adding inadvertent ingestion, the dose was 14.78 mrem/y.

2.1.6 Reviewers’ Comments on NIOSH Bounding Method to Estimate Intakes of Thorium

Assuming that the specific activity of the uranium contamination in the pipe sediments was that 

of natural uranium is not valid. According to Weston (1996, Table 1), the uranium isotopic ratios 

in the pipes indicated that in the 20 samples listed, 15 show enrichments of 1.5% to 35%, while 

two samples list only 235U activity concentrations and no enrichment data. The remaining three 

samples had enrichments <0.72%, the relative abundance of 235U in natural uranium. We thus 

conclude that most of the pipe sediments were contaminated with uranium of varying 

enrichments. It is therefore difficult to assign a single specific activity of uranium to these 

samples.

Furthermore, there is no straightforward method of calculating a ratio of uranium-to232Th 

activity from the Sowell (1985) data. These data list activity concentrations of only two uranium 

isotopes: 235U and 238U—there are no data on 234U. Examining the only two data sets that include 

a substantial number of samples with 232Th levels significantly elevated above background, we 

find that the average 235U activity concentrations in Table 5A to be 24% of the 238U 

concentrations, while the average 235U concentrations in Table 6A are 12% of those of 238U. 

Since the 235U activity in natural uranium is 4.6% of 238U, one cannot assume the isotopic 

abundance of natural uranium to estimate the missing 234U data nor the total uranium activities in 

these soil samples. 

2.1.7 Alternate Method for Estimating 232Th Intakes by Workers Performing Subsurface 

Maintenance Inside Building 10

To overcome the difficulties discussed above, we propose an alternate method of estimating 

232Th intakes by workers performing subsurface maintenance inside Building 10. Since the 

volume-weighted average of the 235U activity concentrations in the samples from the pipes under 

Building 10 (Weston 1996, Table 1) is almost twice that of 238U, and since there are 20 activity 

measurements of this isotope vs. 18 of 238U, the 235U activity concentrations potentially 

constitute a more robust data set for estimating the corresponding 232Th activities. To derive a 

95th percentile 235U activity concentration, we ranked the activity concentrations in the 20 

samples, shown in the first column of Table 2 of the present memo. The second column lists the 

volume of the residue corresponding to each sample (Weston 1996, Table 5). The third column 
Memo – M&C Thorium and Welding Exposure Model 5 SC&A – July 26, 2019

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lists the cumulative volume: each entry is the sum of the corresponding entry in column 2 plus 

all the previous entries in that column. We calculated the 95th percentile volume as 881,184 mL 

(927,562 mL × 0.95 = 881,184 mL). We then used linear interpolation to derive the 95th percentile 

235U concentration (i.e. the concentration corresponding to 95% of the cumulative volume) of 

1,529pCi/g. 

Table 2. U-235 Activity Distribution in Pipe Sediments under Building 10

U-235

(pCi/g)

Volume

(mL)

CVD a

(mL)

0.15 1,977 1,977

0.80 16,062 18,039

0.84 62,087 80,126

1.1 6,178 86,304

1.4 8,031 94,335

1.5 37,067 131,401

1.6 2,471 133,872

1.8 4,633 138,506

2.6 16,371 154,877

4.2 37,067 191,944

5.4 32,124 224,068

13 4,633 228,701

17 24,248 252,949

21 6,178 259,127

21 200,160 459,287

30 43,553 502,840

36 37,067 539,907

53 123,556 663,462

193 86,180 749,642

2000 177,920 927,562

Source: Weston 1996

a Cumulative volume distribution: sum of corresponding 

entry in col. 2 plus all the previous entries in that column

Sowell (1985, Table 5A), “Radionuclide Concentrations in Soil Samples Collected from 

Locations of Elevated Contact Radiation Levels - Building 12 Burial Area,” reports 235U and 

232Th concentrations in samples from 18 locations at depths varying from 2 to 40 cm. To use the 

235U activity concentrations in this table to predict the 232Th levels in the pipes, we needed to 

establish a relationship between the two sets of data. We calculated the correlation coefficient, R, 

between the 235U activity concentrations in these 18 samples and the 232Th concentrations in the 

same samples and derived a value of R = -0.16. The negative correlation indicates that high 235U 

concentrations are more likely to coincide with low 232Th levels, and vice versa. We therefore 

concluded that the data in Table 5A are not a suitable basis for estimating 232Th intakes based on 

measured 235U activities.

The remaining data set is Sowell (1985, Table 6A). Table 6A, “Radionuclide Concentrations in 

Borehole Soil Samples from Building 12 Burial Area,” lists 235U and 232Th concentrations in 88 

core samples taken from 25 boreholes. For the 235U concentrations that were listed as less than a 
Memo – M&C Thorium and Welding Exposure Model 6 SC&A – July 26, 2019

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given value, we assigned one-half that value to the given sample. The correlation coefficient 

between the 235U activity concentrations in these samples and the 232Th concentrations in the 

same samples, R, equals 0.0969. The probability that this correlation is due to chance is 18.5%. 

Although the correlation is not as strong as might be desired, we nevertheless conclude that these 

data constitute the best available basis for using uranium activities to estimate 232Th intakes.1

 

1 Comparisons of 232Th concentrations to those of 238U or to the sums of the 235U + 238U concentrations 

yielded lower values of R. 

We calculated the ratios of the 232Th concentrations to the corresponding 235U concentrations in 

the 88 samples described by Sowell (1985, Table 6A)2

 and derived a geometric mean of 4.062 

from these 88 ratios. Applying this ratio to the 95th percentile 235U concentration of 1,529 pCi/g in 

the pipe sediments, we obtained a 232Th concentration of 6,211 pCi/g in the pipe residues. 

Applying the 95th percentile dust loading of 2.2 × 10-4 g/m3 cited by McCloskey and Sharfi 

(2018), we obtained an air concentration 1.366 pCi/m3. Assuming that workers were exposed to 

this activity concentration for one month each year, we estimated an inhaled intake of 275 pCi 

(10.2 Bq) per year, which led to an effective dose of 53.7 mrem/y from this pathway. According 

to OCAS-TIB-009 (Neton 2004, p. 4), the daily ingestion rate “can be approximated by 

assuming it to be 0.2 times the activity per cubic meter of air.” Applying this guidance, we obtain 

an effective dose from ingestion of 232Th of 0.02 mrem/y. Our total effective dose from both 

pathways is more than 3-fold higher than the dose of 14.78 mrem/y cited by McCloskey and 

Sharfi (2019). 

2 Sowell (1985, Table 6A) actually presents results for 89 samples, but concentrations of 235U and 232Th are 

shown in only 88 cases. 

Finding 1: NIOSH underestimated the 232Th concentration in the sediments and residues in the 

pipes under Building 10, leading to an underestimate of 232Th intakes by workers performing 

subsurface activities.

2.2 Internal Exposures from Welding

McCloskey and Sharfi (2019) described the NIOSH bounding method for estimating internal 

exposures from welding during the residual period at M&C. The scenario applies only to 

maintenance workers who performed welding tasks. The authors estimated that workers spent 

approximately 4 h per month on welding activities. We reviewed the reference cited for this 

estimate (ORAUT 2017) and could not find any discussion that documented that assumption. 

Observation 2: NIOSH should clarify the source of the 4-h-per-month time estimate.

In calculating the airborne activity concentration, McCloskey and Sharfi (2019) adopted a 

resuspension factor (RF) of 10-4, then increased it by a factor of 10 to incorporate a dispersibility 

factor cited in NUREG-1400 (Hickey, et al. 1983). The authors used these values to calculate an 

airborne concentration of 4.05 × 10-12 μCi/mL gross alpha. Furthermore, based on an annual 

exposure duration of 48 h, and assuming the entire activity to be 234U, they calculated an 
Memo – M&C Thorium and Welding Exposure Model 7 SC&A – July 26, 2019

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effective dose delivered by the inhalation pathway to a worker performing welding of 

5.88 mrem. They calculated an additional dose of 16.77 mrem/y from 232Th and its progeny.

We believe that the highly dispersive nature of the activities accompanying welding—grinding 

and wire brushing to achieve a clean surface—should be modeled using the highest reported RF 

in an indoor environment. According to OTIB-0070 (Sharfi 2012, Table 3-1), “vigorous 

sweeping by two workmen” resulted in RFs of 1.02 × 10-2 to 4.2 × 10-2

. We recommend a 

rounded value of 10-2

, at the lower end of the range, since it is unlikely that the grinding and 

brushing occupied the entire time the worker was involved in the welding activities. This would 

lead to a 10-fold increase in the inhaled intakes estimated by McCloskey and Sharfi (2019).

Finding 2: NIOSH understated the resuspension factor related to activities accompanying 

welding.

We verified that the airborne activity concentration and the effective dose from 234U via 

inhalation reported by McCloskey and Sharfi (2019) were consistent with their stated surficial 

activity concentrations, RF, and exposure duration. However, we do not understand the basis for

the additional dose from the inhalation of 232Th: If all the alpha activity were due to 234U, there 

would have been no thorium. Since there does not appear to be any straightforward means of 

apportioning the surficial activity among the uranium and thorium isotopes, the activity should 

be assigned to whichever radionuclide results in the highest dose in a given case. This would be 

consistent with the authors’ statement (p. 5): “For those areas where gross alpha contamination 

surveys are available, NIOSH will continue to estimate worker doses using the most claimantfavorable isotope of thorium or uranium.” Since the intakes in the welding scenario are based on 

the 95th percentile value derived from gross alpha contamination surveys, this procedure should 

be applied.

Observation 3: In estimating doses from the welding scenario, NIOSH should assign doses using 

the most claimant-favorable isotope of thorium or uranium, selected from isotopes known to 

have been used at M&C.

3 Summary and Conclusions

We find that NIOSH has developed plausible approaches to modeling exposures of M&C

workers to residual 232Th contamination and to modeling work activities related to welding. We 

disagree with some of the parameters and assumptions that NIOSH used to implement its 

approach. However, we believe that these issues can be resolved. These therefore constitute site 

profile rather than SEC issues.
Memo – M&C Thorium and Welding Exposure Model 8 SC&A – July 26, 2019

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4 References

ASTRA, Inc. 1962. “Analysis of Possible Nuclear Material Losses and Possible Liabilities 

Associated with Present Fuel Manufacturing at Metals & Controls, Inc., Attleboro, 

Massachusetts,” ASTRA 410-6.2. SRDB Ref. ID: 013612. 

Creative Pollution Solutions, Inc. (CPS). n.d. “Radiological Characterization of Texas 

Instruments, Incorporated, Attleboro Industrial Facility, Attleboro, Massachusetts.” SRDB Ref 

ID: 164755. 

Hickey, E. E., et al. 1983. “Air Sampling in the Workplace,” NUREG-1400, Final Report. 

Washington, DC: U.S. Nuclear Regulatory Commission. SRDB Ref ID: 20129.

McCloskey, P., and M. Sharfi. 2018. “Metals and Controls Corp. Maintenance Worker Exposure 

Model,” White Paper. SRDB Ref ID: 174357. 

McCloskey, P., and M. Sharfi. 2019. “Metals and Controls Corp. Thorium and Welding 

Exposure Model,” White Paper. 

Neton, J. W. 2004. “Estimation of Ingestion Intakes,” OCAS-TIB-009, rev. 0. 

Oak Ridge Associated Universities Team (ORAUT). 2017. Interview Worker #7; documented 

communication with former Metals and Controls Corporation worker; October 25, 2017. SRDB 

Ref ID: 169938. 

Roy F. Weston, Inc. (Weston). 1996. “Texas Instruments Incorporated, Attleboro Facility: 

Building Interiors Remediation Drainage System Characterization.” SRDB Ref ID: 165965. 

Sharfi, M. M. 2012. “Dose Reconstruction during Residual Radioactivity Periods at Atomic 

Weapons Employer Facilities,” ORAUT-OTIB-0070, Rev. 01. 

Sowell, L. L. 1985. “Radiological Survey of the Texas Instruments Site Attleboro, 

Massachusetts,” RSAP/SMP5-8. SRDB Ref. ID: 94371.